It is to be understood that the present invention is equally applicable in the context of generators as well as motors. However, to simplify the description that follows, reference to a motor should also be understood to include generators.
In the field of electric machine rotor cores, stator cores and generators, the machine cores are typically constructed using laminations stamped from electrical steel. The laminations are stacked and pressed onto a shaft. Then, in most electric machines, windings or permanent magnets are added. These laminations are configured to provide a machine having magnetic, non-magnetic, plastic and/or permanent magnet regions to provide the flux paths and magnetic barriers necessary for operation of the machines. When the shape of the laminations and/or the additional winding/permanent magnet components are compromised, reduced operating speed and flux leakage may occur, thus limiting performance of the electric machine. By way of example, synchronous reluctance rotors formed from stacked axial laminations are structurally weak due to problems associated both with the fastening together of the laminations and with shifting of the laminations during operation of their many circumferentially discontinuous components. This results in a drastically lower top speed. Similarly, stamped radial laminations for synchronous reluctance rotors require structural support material at the ends and in the middle of the magnetic insulation slots. This results in both structural weakness due to the small slot supports and reduced output power due to magnetic flux leakage through the slot supports. There are various types of machines utilizing rotors that require non-magnetic structural support, including synchronous reluctance machines, switched reluctance machines, induction machines, surface-type permanent magnet machines, circumferential-type interior permanent magnet machines, and spoke-type interior permanent magnet machines. Each of these machines utilize rotor components or rotor sense rings of composite magnetic, non-magnetic, plastic, electric and/or permanent magnet materials that suffer from the aforementioned problems.
Despite the aforementioned problems, and the general acceptance of conventional lamination practices as being cost effective and adequate in performance, new powder metal manufacturing technologies can significantly improve the performance of electric machines by bonding magnetic (permeable) and non-magnetic (non-permeable) materials together. Doing so permits the use of completely non-magnetic structural supports that not only provide the additional strength to allow the rotors to spin faster, for example up to 80% faster, but also virtually eliminate the flux leakage paths that the traditionally manufactured electric machines must include to ensure rotor integrity, but which lead to reduced power output and lower efficiency.
Powder metal manufacturing technologies that allow two or more powder metals to be bonded together to form a rotor core have been recently disclosed by the present inventors. Specifically, the following co-pending patent applications are directed to composite powder metal electric machine rotor cores fabricated by a compaction-sinter process: U.S. patent application Ser. No. 09/970,230 filed on Oct. 3, 2001 and entitled “Manufacturing Method and Composite Powder Metal Rotor Assembly for Synchronous Reluctance Machine”; U.S. patent application Ser. No. 09/970,197 filed on Oct. 3, 2001 and entitled “Manufacturing Method And Composite Powder Metal Rotor Assembly For Induction Machine”; U.S. patent application Ser. No. 09/970,223 filed on Oct. 3, 2001 and entitled “Manufacturing Method And Composite Powder Metal Rotor Assembly For Surface Type Permanent Magnet Machine”; U.S. patent application Ser. No. 09/970,105 filed on Oct. 3, 2001 and entitled “Manufacturing Method And Composite Powder Metal Rotor Assembly For Circumferential Type Interior Permanent Magnet Machine”; and U.S. patent application Ser. No. 09/970,106 filed on Oct. 3, 2001 and entitled “Manufacturing Method And Composite Powder Metal Rotor Assembly For Spoke Type Interior Permanent Magnet Machine,” each of which is incorporated by reference herein in its entirety. Additionally, the following co-pending application is directed to composite powder metal electric machine rotor cores fabricated by metal injection molding: U.S. patent application Ser. No. 09/970,226 filed on Oct. 3, 2001 and entitled “Metal Injection Molding Multiple Dissimilar Materials To Form Composite Electric Machine Rotor And Rotor Sense Parts,” incorporated by reference herein in its entirety. Both the compaction-sinter process and the metal injecting molding process (as disclosed in the above-referenced patent applications) lead to the advantages described above, such as strong structural support and virtually non-existent permeable flux leakage paths, and do provide an opportunity to manufacture an electric machine that costs less, spins faster, provides more output power, and is more efficient.
In the compaction-sinter process described in the above-identified co-pending applications, the magnetic and non-magnetic metal powders are poured into respective sections of a disk-shaped die insert. Upon removal of the die insert, the powders, after some settling and mixing along their boundaries, are compressed to a “green” strength, which is usually on the order of 2-6 ksi (13.8-41.4 MPa). The green part is then sintered, such as at about 2050° F. (1121° C.), for about one hour to obtain full strength, typically on the order of 30-50 ksi (207-345 MPa). One disadvantage of this compaction process is that the mixing that occurs after the die insert is removed can lead to blurred boundaries between permeable and non-permeable materials thereby reducing performance. Further, the blurring of boundaries is often particularly pronounced near the top and bottom of the pressed disks such that these sections of the machine do not adequately perform their intended function. To overcome this disadvantage, approximately one-third to two-thirds of the disk's thickness is ground away to leave a middle section having minimal blurring of boundaries that can be effectively utilized as an electric machine component.
The composite metal injection molding process described in the above-identified co-pending application does not exhibit the problem of boundary blurring like the composite compaction-sintering manufacturing process because the magnetic and non-magnetic materials are injection-molded separately into molds that provide definitive edges. However, the injection molding process can be expensive because liquifying the metals generally requires the use of powders that are more expensive and of finer grain size than the powders that can be used in the compaction process. Thus, composite metal injection molding may not be cost effective for a broad range of electric machine applications.
There is thus a need to provide a powder metallurgy manufacturing process that is cost effective and provides definitive boundaries between magnetic (permeable) and non-magnetic (non-permeable) portions of the electric machine components.